1. Field of the Invention
The present invention relates to a stacked type piezoelectric element usable as a source of drive force for an injector.
2. Description of the Related Art
An injector (fuel injection device) of an internal combustion engine of an automobile etc. is designed to inject fuel by operating a valve element of a three-way valve or two-way valve connected to a common rail storing high pressure fuel to switch the opening state of the fuel passages and change the state of pressure imparted to the nozzle needle to open the needle nozzle.
A solenoid valve etc. is generally used as the source of drive force for operating the valve element. Recently, however, attempts have been made to use a stacked type piezoelectric element as the source of drive force, as disclosed in for example Japanese Unexamined Patent Publication (Kokai) No. 11-229993, for the purpose of finer control of the source of drive force and precision control of the state of fuel injection.
While injectors using piezoelectric elements as sources of drive force have been proposed as in Japanese Unexamined Patent Publication (Kokai) No. 11-229993, none have yet been commercialized.
Depending on the type of injector, the injection of a high pressure fuel of for example over 100 MPa is required. Therefore, reliability under a tough environment of use is required in a piezoelectric element for an injector.
When using a conventional piezoelectric element utilized in other fields for an injector of an internal combustion engine of an automobile etc., the piezoelectric element has often not been able to withstand practical use such as, for example, cracking of the piezoelectric element, disconnection in the internal electrode layers, short-circuits due to cracking, etc. in a short time under such a tough environment of use.
Further, or such high pressure fuel injection, a piezoelectric element having an extremely high performance of a displacement of at least 20 μm and a generated force of at least 1000N is necessary. From this viewpoint as well, it has been difficult to use conventional piezoelectric elements for such injectors.